This application is related to Japanese application No. 2001-093113 filed on Mar. 28, 2001 whose priority is claimed under 35 USC xc2xa7 119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to an optical data recording medium on which data is recorded and from which data is reproduced, and more particularly, it relates to an optical data recording medium capable of preventing its warp caused by a change in ambient conditions.
2. Description of Related Art
FIG. 1 is a schematic sectional view illustrating a structure of an optical data recording medium. A conventional optical data recording medium is shown in a plan view and a side view of FIGS. 8(a) and 8(b), respectively.
An optical data recording medium comprises, as shown in FIG. 1, a single layered or multilayered thin film layer 40 including at least any one of dielectric films 41, 43 (silicon nitride), a recording film 42 (TbFeCo) and a reflective film 44 (Al) is formed by sputtering or the like on a disc-shaped substrate 20 made of a polycarbonate. On the thin film layer 40, a protective film 50 such as a resin film for protecting the thin film layer is formed. Further, another protective film 30 such as a resin film for protecting the substrate is formed on a light receiving surface of the substrate.
The substrate 20 is about 1.2 mm thick, the single layered or multilayered thin film layer 40 formed by sputtering is 10-300 nm thick, the protective film 50 is 1-30 xcexcm thick, and the protective film 30 is 0.1-30 xcexcm thick.
Since the polycarbonate substrate 20 constitutes almost the entire thickness of the optical data recording medium, rigidity of the medium substantially depends on that of the polycarbonate substrate 20. With the sufficient thickness of the polycarbonate substrate 20, deformation of the medium caused by a change in ambient conditions (temperature and humidity) is very small and there is no need to pay attention to a balance between stresses and bending moments generated in the layers.
However, in recent years, high density recordation and reproduction on and from the optical data recording medium are demanded, and there is a tendency that attempts to increase NA of an objective lens and to decrease the thickness of the substrate are made to decrease a diameter of a beam spot for realizing high density recordation and reproduction. In particular, there is a tendency that the thickness of the substrate is decreased from 1.2 mm, which is a conventional value, to a half or less of the conventional value, i.e., 0.6 mm and 0.5 mm. In the case where the substrate becomes thin, the rigidity of the optical data recording medium 10 is reduced accordingly.
When the rigidity is reduced, distortion caused by stress due to environmental change (temperature and humidity change) in respective layers constituting the optical data recording medium 10 is increased, and such a problem occurs that recordation and reproduction of data becomes difficult.
Therefore, even in the case where the thickness of the transparent substrate 20 is decreased to reduce the rigidity, it is demanded that distortion of the substrate is suppressed as far as possible, so as to provide an optical data recording medium having exhibiting high performance in resistance to environmental change.
As a method for controlling deformation of an optical data recording medium, Japanese Patent Publication No. Hei 4(1992)-195745 proposes a method, in which a dielectric film for preventing warpage is provided on the back surface of the substrate (i.e., the surface on the side where the thin film layer is not formed).
FIG. 5 is a cross sectional view showing the constitution of the conventional optical data recording medium. In FIG. 5, the same symbols are attached to the same parts as in FIG. 1. As shown in FIG. 5, a dielectric layer 60 is provided on the light receiving surface to equate the expansion coefficients of the thin film layer 40 and the dielectric layer 60 on both sides of the transparent substrate 20, whereby a symmetric structure with respect to the transparent substrate 20 is created in the optical data recording medium to prevent warpage of the optical data recording medium.
Japanese Patent Publication No. Hei 11(1999)-16211 discloses an optical data recording medium having the same structure as shown in FIG. 1, in which a substrate protective film having a rigidity higher than that of the substrate is formed to reduce deformation upon environmental change.
Japanese Patent Publication No. Hei 4(1992)-364248 discloses a data recording medium aiming prevention of warpage caused by humidity change. FIG. 6 is a cross sectional view showing the constitution of the data recording medium disclosed in Japanese Patent Publication No. Hei 4(1992)-364248.
The recording medium contains a thin film protective film 50, a thin film layer 40, a substrate 20 and a substrate protective film (dielectric layer) 30, and furthermore, in order to prevent warpage due to humidity change, a moisture permeation preventing film 70 formed with SiO2 or AlN is provided between the substrate 20 and the substrate protective film 30.
Japanese Patent Publication No. 2000-311381 discloses an optical data recording medium having the structure as shown in FIG. 1 containing a thin film protective film 50, a thin film layer 40, a substrate 20 and a substrate protective film 30, in which the substrate protective film 30 has a moisture permeability smaller than that of the thin film protective film 50, whereby warpage caused by moisture change is reduced.
In the above-mentioned conventional recording media according to both of the publications Japanese Patent Publication No. Hei 4(1992)-195745 and Japanese Patent Publication No. Hei 4(1992)-364248, the dielectric layer (30, 60) must be formed by sputtering or the like on the light receiving surface of the substrate. Accordingly, in the manufacture thereof, the thin film layer 40 is formed on a surface of the substrate and then the substrate is turned over to form the dielectric layer (60, 30) on an opposite surface. Therefore, the manufacture is complicated and the charge of the manufacture facility is raised, which increases the manufacture cost.
In the method disclosed in Japanese Patent Publication No. Hei 11(1997)-16211, it is necessary to form SiO2 as the substrate protective film on the substrate through plasma CVD, and thus the same problem arises as in Japanese Patent Publication No. Hei 4(1992)-195745.
In the method disclosed in Japanese Patent Publication No. Hei 4(1992)-364248, it is necessary to form AlN or SiO2 on the light receiving surface of the substrate through sputtering, and thus the same problem arises as in Japanese Patent Publication No. Hei 4(1992)-195745.
In the method disclosed in Japanese Patent Publication No. 2000-311381, when a substrate 20 formed with a transparent resin, such as an acrylic series, a polyolefin series and polycarbonate, is used, the thin film protective film 50 is formed with a resin that suffers expansion with humidity more than the substrate 20 in most cases, and thus such a problem arises that the warpage phenomenon cannot be completely prevented.
The invention relates to, as one aspect, an optical data recording medium comprising a transparent substrate, a thin film layer formed on one surface of the transparent substrate, a thin film protective film comprising a resin as a main component formed on the thin film layer, and a substrate protective film comprising a resin as a main component formed on the other surface of the transparent substrate than the thin film protective film, wherein the thin film layer is a single layer film or a multi-layer film comprising at least one of a dielectric film, a recording film and a reflective film, and the substrate protective film has a coefficient of humidity expansion that is larger than a coefficient of humidity expansion of the thin film protective film.
According to the configuration, deformation (warpage) upon change in humidity can be effectively suppressed.